I. Related Applications
This application claims priority from International Patent Application Serial No. US99/03683, filed Feb. 19, 1999, which is pending and which claims priority from U.S. Provisional Patent Application Serial No. 60/075,211, filed Feb. 19, 1999.
II. Field of the Invention
The present invention relates to communications in general, and to synchronized power control in a multiple access communications system in particular.
III. Description of the Related Art
The use of code division multiple access (CDMA) modulation techniques is but one of several techniques for facilitating communications in which a large number of system users are present. Although other techniques such as time division multiple access (TDMA), frequency division multiple access (FDMA) and AM modulation schemes such as amplitude companded single sideband (ACSSB) are known, CDMA has significant advantages over these other modulation techniques. The use of CDMA techniques in a multiple access communications system is disclosed in U.S. Pat. No. 4,901,307, entitled xe2x80x9cSPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS,xe2x80x9d assigned to the assignee of the present invention and the disclosure of which is incorporated by reference herein.
In the patent just mentioned, a multiple access technique is disclosed wherein a large number of mobile telephone system users, each having a transceiver, communicate through satellite repeaters or terrestrial base stations (also known as cell base stations, or cell-sites) using CDMA spread spectrum communication signals. In using CDMA communications, the frequency spectrum can be reused multiple times thereby permitting an increase in system user capacity. The use of CDMA techniques results in a much higher spectral efficiency than can be achieved using other multiple access techniques.
In conventional cellular telephone systems that use analog FM modulation, the available frequency band is divided into channels typically 30 KHz in bandwidth. The system service area is divided geographically into cells of varying size. The available frequency channels are divided into sets with each set usually containing an equal number of channels. The frequency sets are assigned to cells in such a way as to minimize the possibility of co-channel interference.
Handoff schemes in conventional cellular systems are intended to allow a call or other type of connection for example a data link, to continue when a mobile station crosses the boundary between two cells. The handoff from one cell to another is initiated when the receiver in the cell base station handing off the call or connection notices that the received signal strength from the mobile station falls below a predetermined threshold value. When the signal level falls below the predetermined threshold value, the base station asks the system controller to determine whether a neighboring base station receives the mobile station signal with better signal strength than the current base station.
The system controller responds to the inquiry of the current base station by sending messages to the neighboring base stations with a handoff request. The neighboring base stations then use special scanning receivers to look for the signal from the mobile station on the specified channel. Should one of the neighboring base stations report an adequate signal strength to the system controller, a handoff is attempted.
In a conventional system, a call is discontinued if the handoff to the new base station is unsuccessful. There are many reasons why a handoff failure may occur. For example, if there is no idle channel available in the neighboring cell for communicating the call, handoff fails. Likewise, if a neighboring base station reports hearing the mobile station, but in fact really hears another mobile unit using the same channel in a completely different cell, handoff fails. Handoff can also fail where the mobile station fails to receive a command signal to switch to the new channel in the neighboring cell.
Yet another handoff problem in conventional cellular systems occurs when the mobile unit approaches the border between two cells. In this situation the signal level of the mobile station tends to fluctuate at both base stations, thereby creating a xe2x80x9cping-pongingxe2x80x9d effect. Repeated requests are made to handoff the call back and forth between two neighboring base stations.
In U.S. Pat. No. 5,101,501, entitled xe2x80x9cMETHOD AND SYSTEM FOR PROVIDING A SOFT HANDOFF IN COMMUNICATIONS IN A CDMA CELLULAR TELEPHONE SYSTEM,xe2x80x9d assigned to the assignee of the present invention, the disclosure of which is incorporated by reference herein, a method and system are disclosed for providing communication with the mobile station through more than one cell base station during handoff. In this environment, communication is not interrupted by the handoff from the base station of the cell the mobile station is exiting to the base station of the cell the mobile unit is entering. This type of handoff can be considered a soft handoff in communications between cell base stations because two or more base stations or sectors of a single base station transmit concurrently to the mobile station during the handoff.
An improved soft handoff technique is disclosed in U.S. Pat. No. 5,267,261, entitled xe2x80x9cMOBILE STATION ASSISTED SOFT HANDOFF IN A CDMA CELLULAR COMMUNICATIONS SYSTEM,xe2x80x9d assigned to the assignee of the present invention, the disclosure of which is incorporated by reference herein. In the improved technique of the foregoing patent, the mobile station monitors the signal strength of pilot signals transmitted by neighboring base stations within the system. When the measured signal strength exceeds a given threshold, the mobile station sends a signal strength message to a system controller via the base station through which the mobile station is communicating. Command messages from the system controller to a new base station and to the mobile station establish contemporaneous communication through the new and current base stations. When the mobile station detects that the signal strength of a pilot corresponding to at least one of the base stations through which the mobile unit is currently communicating has fallen below a predetermined level, the mobile station reports the measured signal strength indicative of the corresponding base station to the system controller via the base stations through which it is communicating. Command messages from the system controller to the identified base station and mobile station terminate communication through the corresponding base station while communications through the other base station or stations continue.
A typical cellular or personal communication system also contains some base stations within a cell having multiple sectors. A multi-sectored base station comprises multiple independent transmit and receive antennas or transceivers, each of which covers an area that is less than the total coverage area for the base station. However, the coverage areas of the individual sectors within the cell are not mutually exclusive, and typically there are areas within the cell where the sectors overlap. In general, a cell is divided into sectors to reduce the total interference power to mobile units located within the cell. The use of sectors also increases the number of mobile units that can communicate through the single base station.
The method of soft handoff between neighboring base stations described above can also be applied to a sectorized base station as disclosed in U.S. Pat. No. 5,625,876, entitled xe2x80x9cMETHOD AND APPARATUS FOR PERFORMING HANDOFF BETWEEN SECTORS OF A COMMON BASE STATION,xe2x80x9d assigned to the assignee of the present invention, the disclosure of which is incorporated by reference herein. Each sector of a common base station is treated as a separate and independent base station. Each sector of the base station combines and decodes multipath signals from a common mobile unit. The decoded data is sent directly to the cellular or personal communication system controller by each sector of the base station. Alternately, the data is compared and selected at the base station and the result is sent to the cellular or personal communication system controller. Thus, in a base station having three sectors S1, S2, and S3, soft handoff between the sectors could occur as follows:
1. The mobile unit can communicate with the base station through a sector S1 transceiver;
2. The mobile unit can detect that the pilot signal strength of the base station sector S2 transceiver exceeds a predetermined threshold value;
3. The mobile unit informs the base station controller through the base station sector S1 transceiver that the pilot signal strength of the base station sector S2 transceiver exceeds the threshold value;
4. The base station controller determines the availability of resources in station sector S2 and sends command signals to the mobile unit through the sector S1 transceiver and the sector S2 transceiver;
5. The mobile unit then begins simultaneous communications with the base station through the base station sectors S1 and S2 transceivers;
6. The base station combines the signals received from the mobile unit through its sector S1 and S2 transceivers until either one or both pilot signal strengths of the sectors falls below a predetermined threshold value and communication through the sector S1 and/or sector S2 transceivers is terminated by the base station controller.
U.S. Pat. Nos. 5,267,261 and 5,383,219, entitled xe2x80x9cFAST FORWARD LINK POWER CONTROL IN A CODE DIVISION MULTIPLE ACCESS SYSTEM,xe2x80x9d assigned to the assignee of the present invention, the disclosure of which is incorporated by reference herein, describes a process that enables a mobile unit to update on a frame-by-frame basis the base station or stations through which it communicates by measuring the rate at which the mobile unit correctly or incorrectly decodes each forward frame.
Problems can arise, however, during soft handoff where a mobile unit communicates with more than one transceiver of a sectorized base station. In this environment, it is probable that not all base station transceivers in active communication with the mobile unit correctly decode the power control data from the mobile station. When this occurs, the gain settings of each base station transceiver in active communication with the mobile unit may not be synchronized or may even diverge. Thus, because each transceiver in a sectorized base station transmits on the same PN channel, when the gain settings of the transceivers in active communication diverge, it becomes more difficult for the mobile unit to properly combine the forward link communication signals it receives from each transceiver of the sectorized base station. Accordingly, some mechanism of keeping the forward frame gain settings of sectorized base station transceivers synchronized during handoff between sectors is needed.
The present invention enables two or more transceivers in a sectorized base station to update the power level at which each transceiver transmits to a mobile station on a forward communication link in cases where the gain settings of the two or more transceivers would otherwise diverge.
In particular, the present invention is directed to a method and apparatus for controlling the transmit power levels of first and second base station transceivers, wherein the first and second base station transceivers are respectively associated with first and second sectors of a cell. The received signal strength of a communication signal arriving at the mobile station is initially determined. A power control value that is based on the received signal strength is then transmitted from the mobile station to the first and second base station transceivers. A first received power control value is next generated by attempting to receive the transmitted power control value at the first base station transceiver, and a second received power control value is generated by attempting to receive the transmitted power control value at the second base station transceiver. A common transmit power value is calculated at the base station controller for the first and second base station transceivers when the first and second received power control values are unequal. The communication signal is then transmitted from the first and second base station transceivers in accordance with the common transmit power value.
By implementing this technique, the power levels at which transceivers in active communication with a single mobile unit are transmitting may be synchronized, thereby minimizing or eliminating the problems that may occur when a mobile station attempts to combine communication signals arriving from transceivers with divergent power levels.